The present invention relates to an objective for microscopes and, more specifically, to a so-called objective-with-a-corrector-ring that is capable of compensating for aberrational variations which occur when there is a change in the thickness of a cover glass.
When microscopic samples are generally prepared in the biological field, most of them are a so-called sample with a cover glass, in which a specimen is placed on a slide glass and the cover glass is then applied thereon for sealing purposes. The thickness and refractive index of this cover glass are provided by JIS (the Japanese Industrial Standards) and, usually, the reference thickness for design is 0.17 mm. When the thickness of the cover glass departs from the reference value, there is a degradation of the capability of forming images. As the numerical aperture (NA) increases, the influence of such thickness errors increases, correspondingly. With an NA exceeding 0.8, it is required to make use of a so-called corrector ring to set off aberrational variations. With an NA exceeding 0.9, such variations are so large that any objective can no longer be used without a corrector ring mechanism. In particular, there is a great demand for lenses that are not only of high contrast and high resolving power but can also form images with well-enough flatness even in an ultra-wide field so as to improve the efficiencies of photography and microscopic analysis.
Japanese Provisional Patent Publication No. 57-148717 sets forth one typical objective with a corrector ring, which is of 40 magnifications, has an NA of 0.55 and includes a cover glass of 0 to 2 mm in thickness. According to this publication, the objective comprises, in order from the object side, a first lens group of positive refracting power, a second lens group of positive refracting power and a third lens group of positive refracting power and aberrations due to a thickness change of the cover glass are corrected by moving only the second lens group along the optical axis.
Another typical example that is of 60 magnifications, has an NA of 0.7 and uses a cover glass of 0.7 to 1.7 mm in thickness is disclosed in Japanese Provisional Patent Publication No. 59-100409. This objective comprises, in order from the object side, a first lens group that has positive refracting power and converts luminous flux from the object to a converging flux, a second lens group that is movable along the optical axis in said converging flux and has small refracting power and a third lens group of negative refracting power, and aberrations due to a thickness change of the cover glass are corrected by moving only the second lens group along the optical axis.
A further typical example that is of 40 magnifications, has an NA of 0.95 and uses a cover glass of 0.11 to 0.23 mm in thickness is referred to in Japanese Provisional Patent Publication No. 61-275812. This objective comprises, in order from the object side, a first lens group that has positive refracting power and converts luminous flux to a converging flux, a second lens group that uses a cemented lens component including a diverging cemented surface and a cemented lens component including a converging cemented surface and has a composite positive refracting power, and a third lens group that includes a cemented surface and is of negative refracting power, and aberrations due to a thickness change of the cover glass are corrected by moving the second and third lens groups relative to the first lens group.
The technique of Japanese Provisional Patent Publication No. 57-148717, because of having an NA of 0.55, cannot stand comparison with an objective having an NA of 0.93, to which the invention is directed. Under an NA condition of 0.55, the desired goal is somehow achieved, if there are some certain chromatic aberration, curvature of field and coma. However, as the NA increases to 0.93, the allowable amounts of aberrations decrease, because the focal depth decreases, correspondingly. It is a matter of course that with this technique it is impossible to increase the NA.
The technique of Japanese Provisional Patent Publication No. 59-100409 achieves an NA of 0.7, but chromatic aberration and off-axial aberrations are quite insufficient at an NA as high as 0.93.
Japanese Provisional Patent Publication No. 61-275812 mentioned above refers to an objective having an NA increased to 0.95. A problem with this technique is that the correction of coma is not well-enough at an image height expressed by y=9 or more and coma degrades due to a thickness variation of the cover glass.